The Role of the Reticular Activating and the Limbic Systems

The reticular activating system, formed of an afferent and efferent network of interconnecting neurons distributed in the core midbrain, controls conscious alertness and, thus, makes sensory, motor, and visceral perception possible. Changes in sleep patterns with aging may be related to alterations in the level of alertness as manifested by the EEG changes discussed above and shifts in neurotransmitters, primarily serotonin (60,61) (Chapter 6). Serotonin appears to function as an inhibitory neurotransmitter that modulates the effects of light on circadian rhythmicity and regulates several cyclic hormonal secretions (62,63). It is also the precursor of the pineal hormone melatonin, which, in some, has potent sedative and hypnotic (sleep-inducing) activity (Chapter 12). Insomnia, frequent in the elderly, may depend on several factors superimposed on the aging process. Anxiety, depression, and stress that often affect sleep, are prevalent among many elderly and may account for some of the sleep disturbances.

Changes in the reticular activating system with old age may not only modify sleep patterns but also alter alertness and behavior. A decrease in sensory input to the higher brain centers may result either from an electrochemical failure of the reticular formation to receive, integrate, and relay signals to the sensory cortex, or from decrements in peripheral sensory perception (Chapter 8), or from both. Any impairment of sensory input would impair motor responses and behavior, decrements that can be detected in EEG recordings and physiologic responses. Such sensory-motor alterations among the aged may explain their decline in response time, that is, the speed with which one initiates a motor or behavioral reaction to a sensory stimulus. The greatest slowing of performance is seen in demented individuals.

Another brain system affected by old age is the limbic system, which regulates many types of autonomic responses (e.g., blood pressure and respiration) and behavior (e.g., sexual behavior, emotions of rage and fear, and motivation). The limbic system consists of a rim of cortical tissue around the hilus of the cerebral hemispheres and of deep structures such as the amygdala and the hippocampus. These structures are involved in memory, mood, and motivation and are frequently affected in old age.

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